Biosynthesis of biotin in microorganisms. IV. Repression and derepression of (+ -)-biotin synthesis from (+)-desthiobiotin

A mutant of Escherichia coli that requires high concentrations of biotin

A recessive mutation (bir) of E. coli, at 77 min on the standard map, destroys ability of the bacteria to take up biotin or to grow except at high concentrations of biotin. In a biotin prototroph, the bir mutation causes overproduction and excretion of biotin, and derepression of at least one biosynthetic gene (bioD). Growth of the bir mutant is inhibited by avidin or high concentrations of dethiobiotin. Both effects are reversed by biotin.

Biosynthesis of biotin: synthesis of 7,8-diaminopelargonic acid in cell-free extracts of Escherichia coli

Cell-free extracts prepared from a biotin auxotroph of Escherichia coli were active in catalyzing the synthesis of 7,8-diaminopelargonic acid, an intermediate of the biotin pathway, from 7-oxo-8-aminopelargonic acid. The product was identified on the basis of its chromatographic characteristics and its biotin activities for biotin auxotrophs of E. coli. Enzyme activity was determined in a reaction coupled with the desthiobiotin synthetase system, which is required for the conversion of 7,8-diaminopelargonic acid to desthiobiotin, and by measuring the amount of desthiobiotin formed by microbiological assay. The reaction was stimulated by l-methionine and pyridoxal-5'-phosphate. l-Methionine could not be replaced by any other amino acids tested. Pyridoxamine and pyridoxamine-5'-phosphate were as active as pyridoxal phosphate. The enzyme, presumably an aminotransferase, was demonstrable in the parent strain of E. coli and all mutant strains tested with the exception of a strain which is able to grow on diaminopelargonic acid but not on 7-oxo-8-aminopelargonic acid. Furthermore, the enzyme was repressible by biotin. The results were consistent with the hypothesis that the biosynthesis of 7,8-diaminopelargonic acid from 7-oxo-8-aminopelargonic acid is an obligatory step in the biosynthetic pathway of biotin in E. coli.

Regulation of biotin transport in Saccharomyces cerevisiae

The metabolic control of biotin transport in Saccharomyces cerevisiae was investigated. Nonproliferating cells harvested from cultures grown in excess biotin (25 ng/ml) took up small amounts of biotin, whereas cells grown in biotin-sufficient medium (0.25 ng/ml) accumulated large amounts of the vitamin. Transport was inhibited maximally in cells grown in medium containing 9 ng (or more) of biotin per ml. When avidin was added to biotin-excess cultures, the cells developed the ability to take up large amounts of biotin. Boiled avidin was without effect, as was treatment of cells with avidin in buffer. Avidin did not relieve transport inhibition when added to biotin-excess cultures treated with cycloheximide, suggesting that protein synthesis was required for cells to develop the capacity to take up biotin after removal of extracellular vitamin by avidin. Cycloheximide did not inhibit the activity of the preformed transport system in biotin-sufficient cells. The presence of high intracellular free biotin pools did not inhibit the activity of the transport system. The characteristics of transport in biotin-excess cells (absence of temperature or pH dependence, no stimulation by glucose, absence of iodoacetate inhibition, independence of uptake on cell concentration, and nonsaturation kinetics) indicated that biotin entered these cells by diffusion. The results suggest that the synthesis of the biotin transport system in S. cerevisiae may be repressed during growth in medium containing high concentrations of biotin.

Biosynthesis of biotin in microorganisms. VI. Further evidence for desthiobiotin as a precursor in Eschericia coli

Biotin auxotrophs were isolated from Escherichia coli K-12. One of the mutants was unable to grow on desthiobiotin and accumulated a large amount of a vitamer in medium when growing on an optimal concentration of biotin. The production of the vitamer was inhibited in the presence of an excess amount of biotin. The vitamer was identified as desthiobiotin on the basis of biological activities, avidin combinability, and chromatographic characteristics. The mutant lacked the ability to convert desthiobiotin to biotin. These results further support the hypothesis that desthiobiotin is a normal intermediate in the biosynthesis of biotin in E. coli.